1. Field of the Invention
The present invention relates to NO.sub.x reduction, more particlularly to reduction of NO.sub.x by NO.sub.x trap technology, and more particularly to systems for decomposing NO.sub.x to N.sub.2 and other benign gases in oxygen-rich environments.
2. Description of Related Art
The control of NO.sub.x emissions from vehicles is a worldwide environmental problem. Gasoline engine vehicles can use newly developed three-way catalysts to control such emissions, because their exhaust gases lack oxygen. But so-called "lean-burn" gas engines, and diesel engines too, have so much oxygen in their exhausts that conventional catalytic systems are effectively disabled. Lean-burn, high air-to-fuel ratio, engines are certain to become more important in meeting the mandated fuel economy requirements of next-generation vehicles. Fuel economy is improved since operating an engine stoichiometrically lean improves the combustion efficiency and power output. But excessive oxygen in lean-burn engine exhausts can inhibit NO.sub.x removal in conventional three-way catalytic converters. An effective and durable catalyst for controlling NO.sub.x emissions under net oxidizing conditions is also critical for diesel engines.
Catalysts that promote the reduction of NO.sub.x under oxygen-rich conditions are generally known as lean-NO.sub.x catalysts. Difficulty has been encountered in finding lean-NO.sub.x catalysts that have the activity, durability, and temperature window required to effectively remove NO.sub.x from the exhaust of lean-burn engines. Prior art lean-NO.sub.x catalysts are hydrothermally unstable. A noticeable loss of activity occurs after relatively little use, and even such catalysts only operate over very limited temperature ranges.
Such catalysts that can effectively decompose NO.sub.x to N.sub.2 and O.sub.2 in oxygen-rich environments have been the subject of considerable research. (For instance, see, U.S. Pat. No. 5,208,205, issued May 4, 1993, to Subramanian, et al.) One alternative is to use catalysts that selectively reduce NO.sub.x in the presence of a co-reductant, e.g., selective catalytic reduction (SCR) using ammonia as a co-reductant.
However, another viable alternative involves using co-existing hydrocarbons in the exhaust of mobile lean-burn gasoline engines as a co-reductant and is a more practical, cost-effective, and environmentally sound approach. The search for effective and durable SCR catalysts that work with hydrocarbon co-reductants in oxygen-rich environments is a high-priority issue in emissions control and the subject of intense investigations by automobile and catalyst companies, and universities, throughout the world.
In the presence of hydrocarbons, catalysts that selectively promote the reduction of NO.sub.x under oxygen-rich conditions are known as lean-NO.sub.x catalysts, and more specifically--SCR lean-NO.sub.x catalysts. Selective catalytic reduction is based on the reaction of NO with hydrocarbon species activated on the catalyst surface and the subsequent reduction of NO.sub.x to N.sub.2. More than fifty such SCR catalysts are conventionally known to exist. These include a wide assortment of catalysts, some containing base metals or precious metals that provide high activity. Unfortunately, just solving the problem of catalyst activity in an oxygen-rich environment is not enough for practical applications. Like most heterogeneous catalytic processes, the SCR process is susceptible to chemical and/or thermal deactivation. Many lean-NO.sub.x catalysts are too susceptible to high temperatures, water vapor and sulfur poisoning (from SO.sub.x). As an example, the Cu-zeolite catalysts deactivate irreversibly if a certain temperature is exceeded. Catalyst deactivation is accelerated by the presence of water vapor in the stream and water vapor suppresses the NO reduction activity even at lower temperatures. Also, sulfate formation at active catalyst sites and on catalyst support materials causes deactivation. Practical lean-NO.sub.x catalysts must overcome all three problems simultaneously before they can be considered for commercial use.
In the case of sulfur poisoning, some gasoline can contain up to 1200 ppm of organo-sulfur compounds. Lean-NO.sub.x catalysts promote the conversion of such compounds to SO.sub.2 and SO.sub.3 during combustion. Such SO.sub.2 will adsorb onto the precious metal sites at temperatures below 300.degree. C. and thereby inhibits the catalytic conversions of CO, C.sub.x H.sub.y (hydrocarbons) and NO.sub.x. At higher temperatures with an Al.sub.2 O.sub.3 catalyst carrier, SO.sub.2 is converted to SO.sub.3 to form a large-volume, low-density material, Al.sub.2 (SO.sub.4).sub.3, that alters the catalyst surface area and leads to deactivation. In the prior art, the primary solution to this problem has been to use fuels with low sulfur contents.
A second leading catalytic technology for removal of NO.sub.x from lean-burn engine exhausts involves NO.sub.x storage reduction catalysis, commonly called the "lean-NO.sub.x trap." As with SCR lean-NO.sub.x catalysts, the lean-NO.sub.x trap technology can involve the catalytic oxidation of NO to NO.sub.2 by catalytic metal components effective for such oxidation, such as precious metals; however, in the lean NO.sub.x trap, the formation of NO.sub.2 is followed by the formation of a nitrate when the NO.sub.2 is adsorbed onto the catalyst surface. The NO.sub.2 is thus "trapped", i.e., stored, on the catalyst surface in the nitrate form and subsequently decomposed by periodically operating the system under stoiciometrically fuel-rich combustion conditions that effect a reduction of the released NO.sub.x (nitrate) to N.sub.2.
Both lean-NO.sub.x SCR and lean-NO.sub.x -trap technologies have been limited to use for low sulfur fuels because catalysts that are active for converting NO to NO.sub.2 are also active in converting SO.sub.2 to SO.sub.3. Lean NO.sub.x trap catalysts have shown serious deactivation in the presence of SO.sub.x because, under oxygen-rich conditions, SO.sub.x adsorbs more strongly on NO.sub.2 adsorption sites than NO.sub.2, and the adsorbed SO.sub.x does not desorb altogether even under fuel-rich conditions. Such presence of SO.sub.3 leads to the formation of sulfuric acid and sulfates that increase the particulates in the exhaust and poison the active sites on the catalyst. Attempts with limited success to solve such a problem have encompassed, for example, Nakatsuji et al. describing the use of selective SO.sub.x adsorbents upstream of lean NO.sub.x trap adsorbents.
Furthermore, catalytic oxidation of NO to NO.sub.2 is limited in its temperature range. Oxidation of NO to NO.sub.2 by a conventional Pt-based catalyst maximizes at about 250.degree. C. and loses its efficiency below about 100 degrees and above about 400 degrees. Thus, the search continues in the development of systems that improve lean NO.sub.x trap technology with respect to temperature and sulfur considerations.
The U.S. Federal Test Procedure for cold starting gasoline fueled vehicles presents a big challenge for lean-NO.sub.x trap catalysts due to the low-temperature operation involved. Diesel passenger car applications are similarly challenged by the driving cycle that simulates slow-moving traffic. Both tests require reductions of CO, hydrocarbons, and NO.sub.x at temperatures below 200.degree. C. when located in the under-floor position. Modifications of existing catalyst oxidation technology are successfully being used to address the problem of CO and hydrocarbon emissions, but a need still exists for improved NO.sub.x removal.